1. Field of Invention
This invention relates to turbine rotor blades and in particular to rotor blades for use in gas turbine engines.
2. Description of Related Art
The turbine of a gas turbine engine depends for its operation on the transfer of energy between the combustion gases and the turbine. The losses which prevent the turbine from being totally efficient are due at least in part to gas leakage over the turbine blade tips.
Hence the efficiency of each rotor stage in a gas turbine engine is dependent on the amount of energy transmitted into the rotor stage and this is limited particularly in unshrouded blades by any leakage flow of working fluid ie. air or gas across the tips of the blades of the rotors.
In turbines with unshrouded turbine rotor blades a portion of the working fluid flowing through the turbine tends to migrate from the concave pressure surface to the convex suction surface of the aerofoil portion of the blade through the gap between the tip of the aerofoil and the stationary shroud or casing. This leakage occurs because of a pressure difference which exists between the pressure and suction sides of the aerofoil. The leakage flow also causes flow disturbances to be set up over a large proportion of the height of the aerofoil which leads to losses in efficiency of the turbine.
By controlling the leakage flow of air or gas across the tips of the blades it is possible to increase the efficiency of each rotor stage.
There is disclosed in GB 2155558A an unshrouded rotor blade which has a recess at its radially outer extremity. The recess is defined by a peripheral wall and a number of transverse walls extending across the recess, thereby dividing the aerofoil into a number of chambers. These walls form a labyrinth seal and trapped vortices are set up in each of these chambers. The trapped vortices aim to reduce the leakage flow between the tip of the blade and the shroud or casing.
The above arrangement traps the leakage flow within the recesses thereby reducing leakage flow across the tip of the blade. However the kinetic energy of this flow is still lost since it remains trapped within the chambers. This flow still forms a vortex in the main passage, albeit of reduced strength, which generates extra loss. In addition the prior art arrangement suffers from the disadvantage that most of the over tip leakage flow is over the rear part of the aerofoil where typically it is too thin to form within a cavity.